This invention relates to a method and apparatus for measuring the calcium oxalate scale forming propensity of fluids and the effectiveness of calcium oxalate scale inhibitors. More specifically, this invention concerns a method of measuring the rate of calcium oxalate scale deposition on to the surface of a piezoelectric microbalance immersed in the fluid where the scale deposition is driven by an electrochemically controlled pH change in the vicinity of the microbalance.
Calcium oxalate scale is a persistent problem in a variety of industrial processes involving water, such as pulp bleaching and sugar production. The calcium oxalate scale may remain suspended in the water or form hard deposits that accumulate on the surface of any material that contacts the water. This accumulation prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria.
A primary detrimental effect associated with scale formation and deposition is the reduction of the capacity or bore of receptacles and conduits employed to store and convey the water. In the case of conduits used to convey scale-contaminated water, the impedance of flow resulting from scale deposition is an obvious consequence.
However, a number of equally consequential problems arise from utilization of scale-contaminated water. For example, scale deposits on the surfaces of storage vessels and conveying lines for process water may break loose and become entrained in and conveyed by the process water to damage and clog equipment through which the water is passed, e.g., tubes, valves, filters arid screens. In addition, these deposits may appear in, and detract from, the final product derived from the process, such as paper formed from an aqueous suspension of pulp.
Furthermore, when the scale-contaminated water is involved in a heat exchange process, as either the xe2x80x9chotxe2x80x9d or xe2x80x9ccoldxe2x80x9d medium, scale will be formed upon the heat exchange surfaces contacted by the water. Such scale formation forms an insulating or thermal opacifying barrier that impairs heat transfer efficiency as well as impeding flow through the system. Thus, scale formation is an expensive problem in many industrial water systems, causing delay and expense resulting from shutdowns for cleaning and removal of the deposits.
Calcium oxalate scale in biological fluids is another significant problem. In particular, kidney stones are formed of calcium oxalate, and urine analysis for calcium oxalate precipitation are used to assess the susceptibility of a patient to kidney stone formation and to monitor and screen pharmaceutical remedies.
Accordingly, there is an ongoing need for the development of new agents that prevent or inhibit the formation of calcium oxalate scales in fluids and for convenient methods of measuring the effectiveness of these inhibitors. In addition, as natural inhibitors may already be present in the solutions of interest, there is a need for effective methods of characterizing the tendency of industrial and biological solutions as such to form calcium oxalate deposits.
The effectiveness of these calcium oxalate scale inhibitors is manifested by their ability to suppress crystal growth through blocking active sites of potential centers of crystallization and preventing the agglomeration of growing crystals.
Common to the above processes is that they occur at the solid-liquid interface. Therefore the in situ measurement of the rate of crystal growth in the presence calcium oxalate scale inhibitors at the solid-liquid interface is of particular interest. Traditional measurements mostly relate to the change of the bulk properties of a test solution such as solubility, conductivity, turbidity and the like following crystal formation. There exist only a few methods for measuring crystal growth rate, and even fewer methods for conducting the measurements in situ at the solid-liquid interface.
Methods for measuring crystal growth rate at the solid-liquid interface that utilize a piezoelectric microbalance are disclosed in U.S. Pat. Nos. 5,201,215 and 6,250,140 and European Patent Application No. 676 637 A1. The principle of piezoelectric mass measurement is based upon the property of a quartz resonator to change its mechanical resonance frequency f0 proportionally to the mass and viscoelastic properties of the deposited material. The change in frequency is expressed as follows:                               Δ          ⁢                      xe2x80x83                    ⁢          f                ≈                  -                                                    2                ⁢                                  xe2x80x83                                ⁢                                  f                  0                  2                                                                              N                  ⁡                                      (                                                                  μ                        μ                                            ⁢                                              xe2x80x83                                            ⁢                                              ρ                        q                                                              )                                                  ⁢                                  1                  /                  2                                                      ⁡                          [                                                ρ                  s                                +                                                      (                                                                  ρ                        ⁢                                                  xe2x80x83                                                ⁢                        η                                                                    4                        ⁢                                                  xe2x80x83                                                ⁢                        π                        ⁢                                                  xe2x80x83                                                ⁢                                                  f                          0                                                                                      )                                                        1                    /                    2                                                              ]                                                          (        6        )            
where f0 is the unperturbed resonant frequency of the quartz crystal; N is the harmonic number; xcexcxcexcis the quartz shear stiffness, xcfx81q is the density of quartz; xcfx81s is the surface mass density of the deposit (mass/area), xcfx81 is the density of the medium contacting the resonator and xcex7 is the viscosity of the medium contacting the resonator.
Where the viscoelastic properties of the system are negligible or remain constant through the measurements, the surface mass density can be measured using a simplified expression that can be used for the loading causing the resonant frequency change up to 5% (approx. 4.5 mg/cm2):
xcfx81s=xe2x88x92Cxcex94f0
where C is determined by calibration and is typically equal 1.77xc3x9710xe2x88x925 mg/(sec cm2 Hz) for a 5 MHz quartz crystal.
However, as discussed herein, the piezoelectric microbalance described in the foregoing references is unsuitable for testing calcium oxalate solutions as it does not provide the necessary conditions for the calcium oxalate crystals to precipitate on the surface of the microbalance. Consequently, a need still exists for methods of measuring the calcium oxalate scale forming tendencies of solutions under conditions at which calcium oxalate scale forming behavior is exhibited.
We have discovered that a metal-plated quartz-crystal microbalance can be used to provide the necessary conditions for the calcium oxalate crystals to precipitate on the surface of the microbalance, in particular by controlling the solution pH proximate to the surface of the microbalance by applying an appropriate electric polarization to the metal surface (the working electrode).
However, not any material can be used for plating the quartz crystal microbalance. Thus, piezoelectric microbalances utilizing traditional gold-coated crystals cannot be used to test calcium oxalate scale inhibitors as intensive hydrogen evolution is observed at the potential that provides for the near-surface pH suitable for oxalate scale formation. This hydrogen evolution interferes with and often completely precludes deposition of calcium oxalate scale on the microbalance.
Also, the test solution should have a proper pH and concentration of calcium oxalate. The solution pH should be low enough to provide for full solubility of the constituents. However, pH""s less than 2 may be too low for an electrochemical polarization to produce the pH increase at the quartz microbalance sufficient to precipitate calcium oxalate from the solution while avoiding the evolution of hydrogen bubbles. On the other hand, pH""s higher than 3 may not provide for the concentration of calcium and oxalate ions in the bulk solution sufficient for a reasonable deposition rate and rapid completion of the test.
Moreover, the surface activities of the inhibitors as well as the adsorption properties of the deposition interface depend on the pH. In order to keep the screening conditions the same for various solutions an actual knowledge of the pH in the vicinity of the microbalance working electrode is required.
We have developed a method and apparatus for testing potential calcium oxalate scale inhibitors and the capacity of industrial and biological solutions to form calcium oxalate deposits that utilizes a controlled change of the pH in an oxygen-saturated acidic test solution near the deposition substrate represented by the working electrode of a quartz crystal microbalance (QCM).
Accordingly, in its principal embodiment, this invention is directed to a method of measuring the calcium oxalate scale forming propensity of a continuously flowing solution having a pH of from about 2 to about 3 comprising measuring the rate of deposition of calcium oxalate scale from the solution on to a quartz crystal microbalance having a top side comprising a working electrode in contact with the solution and a second, bottom side isolated from the solution, wherein the pH of the solution proximate to the microbalance is controlled electrochemically at from about 3.5 to about 9 and wherein the working electrode is coated with or made of a conductive material on which the intensive evolution of hydrogen gas proceeds at potentials more negative than necessary to achieve a pH of 3.5-9 proximate to the microbalance.
In another aspect, this invention is directed to method of measuring the effectiveness of calcium oxalate scale inhibitors comprising
a) measuring the calcium oxalate scale forming propensity of a continuously flowing solution having a pH of from about 2 to about 3 comprising measuring the rate of deposition of calcium oxalate scale from the solution on to a quartz crystal microbalance having a top side comprising a working electrode in contact with the solution and a second, bottom side isolated from the solution. wherein the pH of the solution proximate to the microbalance is controlled electrochemically at from about 3.5 to about 9 and wherein the working electrode is coated with or made of a conductive material on which the intensive evolution of hydrogen gas proceeds at potentials more negative than necessary to achieve a pH of 3.5-9 proximate to the microbalance;
b) adding a calcium oxalate scale inhibitor to the solution; and
c) re-measuring the rate of deposition of calcium oxalate scale from the solution on to the quartz crystal microbalance.
In another aspect, this invention is directed to an apparatus for measuring the calcium oxalate scale forming propensity of a continuously flowing solution having a pH of from about 2 to about 3 comprising a quartz crystal microbalance having a top side comprising a working electrode for exposure to the solution and a bottom side isolated from the solution, wherein the pH of the proximate to the microbalance is controlled electrochemically at from about 3.5 to about 9 and wherein the working electrode is coated with or made of a conductive material on which the intensive evolution of hydrogen gas proceeds at potentials more negative than necessary to achieve a pH of 3.5-9 proximate to the microbalance.
In another aspect, this invention is directed to apparatus for measuring the calcium oxalate scale forming propensity of a continuously flowing solution having a pH of from about 2 to about 3 comprising a measurement cell with stirring means and mounted in the measurement cell:
a) a quartz crystal microbalance having a top side comprising a working electrode for exposure to the solution and a bottom side isolated from the solution;
b) a surface pH-measuring module for exposure to the solution, the pH-measuring electrode assembly comprising a mesh electrode laid over a pH electrode wherein the mesh is made of the same material as the working electrode of the microbalance;
c) two reference electrodes for exposure to the solution; and
d) two counter electrodes for exposure to the solution, wherein the quartz crystal microbalance and the surface pH-measuring module are mounted horizontally oppositely oriented, the two counter electrodes are mounted vertically and located each at an equal distance and downstream from the quartz crystal microbalance and the surface pH measuring module and the reference electrodes are mounted vertically and located each at an equal distance and downstream from the each of the counter electrodes and wherein the working electrodes of the surface pH measuring module and the quartz crystal microbalance are coated with or made of a conductive material on which the intensive evolution of hydrogen gas proceeds at potentials more negative than necessary to achieve a pH of 3.5-9 proximate to the microbalance.
The method of this invention simulates calcium oxalate scale formation from calcium and oxalate ion-containing solutions under conditions wherein the solution pH is raised above the salt solubility limit, with the solution chemistry providing a characteristic rate of precipitation. The solution pH increase is created electrochemically and controlled in-situ in the vicinity of a metal-plated quartz crystal microbalance which serves as a nucleation plate for the scale crystals.
The method and apparatus of this invention are useful for benchtop laboratory work or, in a portable form, for on-site process control. The method allows reliable and prompt testing of potential calcium oxalate scale inhibitors in both model and real solutions. It is reproducible, sensitive and has broader applications than known techniques that suffer from interference of additional components present in industrial solutions. This method allows specifically characterizing the ability of scale inhibitors to prevent calcium oxalate crystal growth and when used in conjunction with conventional chemical tests allows comprehensive characterization of the properties of calcium oxalate scale inhibitors.
In addition to testing industrial solutions, this method can be applied to biological solutions to characterize their tendency to form calcium oxalate deposits. It has a great potential for medical applications such as urine tests for susceptibility to kidney stone formation and monitoring and screening of potential pharmaceutical remedies.
The method and apparatus of this invention can also be utilized for measuring the inorganic scale-forming propensity of any aqueous solution where solubility of the scale is pH-dependent, including calcium carbonate; calcium salts of organic acids; magnesium hydroxide; and the like.